The most common cause of post-trabeculectomy filtration failure is postoperative scarring. Long-term preoperative treatment with local antiglaucoma agents containing preservatives has an increased risk of post-operative scarring of the filtration bleb. Therefore many surgeons prepare their patients for planned trabeculectomy by discontinuing local antiglaucoma drugs, controlling IOP with oral acetazolamide, and treating the eye with local steroids.